There is a manhole allowing a worker to enter for the maintenance of various city infrastructural facilities such as a drain system, water work facilities, a communication facility, an electric power facilities, etc.
The manhole generally represents a lower body which is installed under the ground, thus communicating the underground space and the ground space. A manhole cover exposed to the ground is mounted at an upper side of the manhole. The manhole cover basically serves to prevent various foreign substances and pedestrians from falling and prevent the bad smell from spreading from the underground space to the ground space by sealing the manhole.
The conventional manhole cover is generally made from a steel casting of a carbon cast steel. The manhole cover made from such materials is heavy, which brings in hard installation and maintenance.
Since the manhole cover made from a cast material is made with a single material, it is easy to recycle the product of the manhole cover to manufacture another product by melting the manhole cover. With its easy recycling to another product, the manhole cover installed on the road or the ground is frequently stolen.
The installation of a certain device for preventing the manhole from being stolen additionally costs a lot. If the manhole is stolen, a big open hole is formed, so a vehicle or a pedestrian might fall though the big open hole. Since the steel cast has a good electrical conductivity, an electric shock accident might occurs in the rain.
A new manhole cover made from a plastic material is developed in an attempt to prevent the above problems of the manhole cover made from a steel cast material; however such new product is too fragile and light as compared to the conventional steel cast product. If a lot of rainwater flows at a time over the manhole cover, the manhole cover might easily escape the installed site.
In order to overcome the problems belonging to a plastic material manhole cover, another new manhole cover is developed, which is made from a plastic-combined steel material; however such manhole cover somehow overcomes the strength and weight problems, but since the steel portions are exposed to the ground, an electric shock problem still remains unsolved.